The present invention relates to an injection unit for an injection moulding machine with a housing carrying a plasticising cylinder and with a transverse support arranged movably in the housing, which transverse support is connected to a plasticising screw longitudinally displaceable and rotatably mounted in the plasticising cylinder.
During the preparation of the plastics material to be processed by means of such an injection unit, the plasticising screw mounted in the plasticising cylinder is put into rotary motion. The plastics material ready for injection then collects in front of the tip of the plasticising screw, wherein In the course of preparation said screw is continuously moved rearwards. In order to inject the prepared plastics into the mould, the plasticising screw in the plasticising cylinder must be moved back again to its initial position. A hydraulic mechanism is normally used for this. It is known to connect to the plasticising screw one or two pistons which can be pressurised on both sides. Such hydraulic mechanisms which are provided with pistons which can be pressurised on both sides with two piston rods are, however, very complex from a constructional viewpoint, On the one hand, three sealing points must be configured, two for the piston rod and one on the piston itself. On the other hand, it is necessary to finish the internal surface of the cylinder, in addition to the two piston rods, for example by honing, in order to obtain the necessary surface quality. In particular in the case of the cylinder, configured as a cast piece, this finishing often gives rise to problems.
The object of the invention Is therefore to constructively improve the hydraulic mechanism of an injection unit so that fabrication is simplified and is thereby more economical.
This is obtained according to the invention in that piston rods projecting on both sides in the longitudinal direction of the injection unit are connected to the transverse support, the free ends of which rods each enter into cylinder chambers in the manner of plungers.
The configuration of the hydraulic mechanism by means of plungers working in opposition results in a reduction of the masses moved during the injection process. In addition, compared to the state of the art, the piston sealing points are eliminated as well as the costly finishing of the internal cylinder surface.
In order to adapt to the different demands of the two directions of movement of the plasticising screw, it is advantageous when the free ends of the piston rods have different diameters. In this way the requirement for a differential system is also satisfied.